1. Field of the Invention
This invention relates to a lens tube of the type having a plurality of aperture blades normally urged in the aperture-opening direction. In particular, the invention relates to such type of lens tube which can be incorporated in a conventional single-lens reflex camera of even the follow-up type.
2. Description of the Prior Art
Conventional lens tubes are of two types: a follow-up type lens tube having a plurality of aperture blades normally urged in the aperture-closing direction, and a pressure type lens tube having a plurality of aperture blades normally urged in the aperture-opening direction.
In the follow-up type lens tube, the aperture blades are placed at the full aperture position when the lens tube is set on the lens mount in the camera. When the shutter button is depressed, the blades are moved by a spring force from the full aperture position to the predetermined aperture position, and then shutter release is made. In this system, the iris diaphragm is closed by the pressure of the spring, and when a predetermined opening is provided, the movement of the aperture blades is halted by a stopper. As a result, vibration of the blades may occur. The vibration is substantial where a large-sized diaphragm is used, because the inertia of the blades is high.
In the pressure type lens tube, the aperture blades are urged in the aperture-opening direction even when it is adjusted with a camera body. When the shutter button is pushed, the blades are turned back from full aperture to the smallest opening position, followed by shutter release. In this case, although movement of the aperture blades is halted by a stopper, the blades are still pressed in the aperture-opening direction by the pressing force of the shutter button even when a desired aperture setting has been achieved, and as a result, little vibration occurs on the blades. The time required for effecting shutter release after attainment of the smallest aperture is shortened. Also, no adverse effect is caused by the inertia of the blades though the camera has a large-sized diaphragm.
However, each type of prior art lens can respectively be used only in connection with the particular type of camera body for which it has been exclusively designed. Also, in order to continuously observe the depth of field, the manual aperture-setting device of the conventional lens tube requires that the photographer push with one hand the aperture-setting button secured to the lens tube or to the camera body while, with the other hand, he turns the aperture ring on the lens tube to adjust the size of the lens opening. This is a troublesome and time-consuming operation.